Rather than having an assistant push your bikes up the ramp, how about fitting a pulley to the ceiling in line with the ramp? That would allow your assistant to pull on a rope - no risk of the assistant tripping and pulling you all off the ramp, and the pull will always be exactly in line with the ramp.

Even better, fit a small electric winch with foot operated on switch at the bottom of the ramp and another on the floor of the truck where you can stop it when the bike is fully inside.